Binnig and Rohrer first successfully demonstrated vacuum tunneling in 1981 and soon thereafter built a vacuum STM which could image topographic features of metal and semiconductor surfaces with resolution on an atomic scale. Previous attempts to construct a device to study surface topography electronically had utilized field emission to produce a current from a tip, which tip was moved or scanned closely adjacent to the surface or sample to be studied. By using tunneling instead of field emission to generate the current, much higher resolution can be attained. If the tip-to-sample voltage is reduced below that which results in field emission and the gap made sufficiently small, a tunneling current flows across the vacuum between the tip and sample. The tunneling current is steeply (exponentially) dependent on the gap width. Thus changes in tunneling current of an order of magnitude can be achieved by a gap change of an Angstrom.
An STM comprises means to establish a tunneling current between a metal tip and a closely adjacent surface to be studied. If the tip is moved relative to and parallel to the surface at a constant distance therefrom, the tunneling current will fluctuate as the tip passes over the hills and valleys of the irregular surface, increasing when a hill is encountered and decreasing when a valley is encountered. By measuring the tunneling current and correlating it with the exponential relationship of this current with the gap, the surface topography can be deduced. In most STMs the tip is scanned parallel to the surface (in the X-Y plane) by means of X and Y piezoelectric transducers (PZTs) and the tip-to-sample distance or gap is controlled by a third Z-axis PZT. A feedback system senses the tunneling current and maintains this current constant by electrically actuating the Z-axis PZT. With this apparatus the tip electrode follows the surface profile of the sample and also the feedback voltage which actuates the Z-axis PZT will vary as does the profile; this voltage can be simply applied to a CRT or to a plotter to obtain a profile display. By the systematic scanning of adjacent lines (as in television scanning), a three-dimensional contour map of the surface can be obtained.
An STM must be free of thermal drift caused by differential expansion of materials with different thermal expansion coefficients, it should be mechanically isolated from its environment, it should not be influenced by internal mechanical resonances, it should operate at high scanning speed, and it may be required to operate under ultra high vacuum (UHV). The large size of early STMs made them susceptible to ambient vibration which necessitated vibration-damping expedients such as elaborate magnetic levitation by means of superconductivity. Present-day STMs have been miniaturized so that mechanical self-resonance is not such a problem.
The prior art includes walking sample holders (or stages) for electron microscopes with which the sample under study can be coarsely adjusted, for positioning, preparatory to the scanning or viewing of a local area thereof. These walking sample holders utilize PZTs with feet attached to the ends thereof. The feet can be selectively electronically clamped to a metallic substrate across which the entire assembly walks. The walking motion occurs in step-by-step fashion. Each step comprises expanding the PZT by means of an applied voltage with the forward foot unclamped and the aft or rear foot clamped to the substrate by means of a voltage applied between the dielectric-coated metallic foot and the conducting substrate. The forward foot is then clamped in a similar fashion and the aft foot unclamped. The PZT voltage is then removed and the resultant contraction of the PZT causes the aft foot to move forward. A repetition of this cycle will cause further movement. Motion in the opposite direction can be achieved by changing the sequence of the clamping and unclamping of the feet. A walking stage or specimen holder of this linear type for a conventional electron microscope is shown and described in the Sakitani Pat. No. 3,952,215, issued on Apr. 20, 1976.
The STM inventors Binnig and Rohrer in an article in the journal Surface Science, Vol. 126 (1983), pp. 236-244 show an STM with X and Y piezodrives connected to the tunnel tip for scanning it parallel to the surface under study and a Z piezodrive connected to vary the tip-to-surface gap. The surface to be studied is supported on a rough or coarse drive system called a "louse", which comprises a piezoelectric plate mounted on three feet which can be electrostatically clamped to a horizontal substrate. Electrically elongating and contracting of the plate with coordinated clamping and unclamping of the feet will move this louse in any direction in the horizontal plane. The sample is supported by the louse in the vertical plane with the tip mounted horizontally adjacent thereto. The gap is varied in the horizontal direction by a PZT on which the tip is mounted. This arrangement only allows coarse positioning along one axis in the sample plane.
In an article entitled High-Stability Scanning Tunneling Microscope, by Van de Walle et al. in the Review of Scientific Instruments, 56(8), August 1985, the authors describe an STM with a scanning (or fine positioning) system comprising a square array of nine stainless steel blocks connected together by means of twelve small piezoelectric cubes (or PZTs). The center block has the tip attached thereto by means of a Z-axis PZT. The four corner steel blocks are fixed and the tip is scanned in the X-Y plane by electrically activating appropriate groups or arrays of the piezoelectric cubes which connect the steel cubes. For example, if one of the linear arrays of three PZTs which are perpendicular to the desired axis of motion is electrically expanded and the other linear array parallel thereto but on the opposite side of the center cube is simultaneously contracted, the center cube and the tip will be moved along the axis perpendicular to the two linear arrays.